The present invention is directed, in general, to power conversion and, more specifically, to a system and method for reducing a DC magnetic flux bias in a transformer and a power converter employing the system or the method.
A power converter is a power processing circuit that converts an input voltage waveform into a specified output voltage waveform. Power converters are typically employed in applications that require conversion of an input DC voltage to various other DC voltages, higher or lower than the input DC voltage. Exemplary applications for power converters include telecommunication and computer systems wherein high voltages are converted to lower voltages to operate the systems.
Current power converter designs often employ a full bridge inverter having four controllable switches (e.g., power metal-oxide semiconductor field-effect transistors), a transformer (e.g., an isolation transformer), an output rectifier and an output filter. A controller is included and employed to control the controllable switches.
A conventional full bridge inverter generally operates as follows. The controllable switches are arranged in two diagonal pairs that are alternately turned on for a portion of a switching period to apply opposite polarities of the input DC voltage across a primary winding of the transformer. The controllable switches thus operate to convert the input DC voltage into an AC voltage required to operate the transformer. Between conduction intervals of the diagonal pairs, all of the controllable switches are turned off for a fraction of the switching period. Ideally, this should result in a voltage across the primary winding of the transformer being substantially zero.
The output rectifier then rectifies the AC voltage from a secondary winding of the transformer. A rectified voltage of the transformer should, therefore, ideally be a square wave with an average value proportional to a duty ratio of the diagonal pairs of the controllable switches. The output filter smooths and filters the rectified voltage to provide a substantially constant output voltage at the output of the power converter. The controller monitors the output voltage and adjusts the duty ratio of the diagonal pairs of the controllable switches to maintain the output voltage at a constant level as the input DC voltage and the load current vary. Alternatively, the controller may monitor the output current and adjust the duty ratio of the diagonal pairs of the controllable switches to maintain the output current at a substantially constant level as the input DC voltage and the load impedance vary.
The transformer may sustain a DC magnetic flux bias as a result of a volt-second imbalance caused by inaccuracies in the control system. A volt-second imbalance between the two half-cycles of each switching cycle implies that a DC voltage component is applied to a core of the transformer. There are several potential causes of the volt-second imbalance including, for instance, an imbalance in the duty cycles of the controllable switches or a small asymmetry in the voltage drops across the controllable switches. Over a number of switching cycles, the continuing increase in the magnetic flux may cause the core of the transformer to saturate, resulting in failure of the power converter employing the transformer.
The transformer may further sustain the DC magnetic flux bias as a result of a physical implementation of a circuit topology, such as an output rectifier topology. In low voltage systems, a hybridge or current-doubler rectifier topology may prove useful. The hybridge rectifier generally includes first and second inductors coupled in series across the secondary winding of the transformer. Unequal resistances of the first and second inductors may result in an unequal division of current between the first and second inductors. A net DC bias may result, with a DC bias voltage (obtained from one end of the first inductor to an opposite end of the second inductor) applied directly across the secondary winding of the transformer. The secondary winding typically has a low resistance, often in the order of milliohms. A difference in the order of millivolts between the first and second inductors, coupled with the low resistance of the secondary winding, may result in amperes of DC current flowing in the secondary winding.
It is therefore advantageous to reduce the DC magnetic flux bias in the transformer to avoid saturation of the core. One common approach to reducing an effect of the DC magnetic flux bias on the transformer is to provide a gap in the core of the transformer. The gap will decrease the magnetizing inductance of the transformer, resulting in an increase in the current flowing in the magnetizing inductance (magnetizing current). The transformer may thus be more tolerant to the DC magnetic flux bias. Interaction of the increased magnetizing current with other currents flowing in the windings of the transformer may increase a power loss in the windings, which may be observed as an increase in an AC resistance of the windings. To reduce the power loss in the windings of the transformer, it may be advantageous to reduce the gap in the core of the transformer. A smaller gap, however, is necessarily more sensitive to the effects of the DC magnetic flux bias.
Accordingly, what is needed in the art is a system and method for reducing a DC magnetic flux bias in a power converter employing a transformer that overcomes the deficiencies of the prior art.
To address the above-discussed deficiencies of the prior art, the present invention provides a system and method for reducing a DC magnetic flux bias in a transformer and a power converter employing the system or the method. The power converter has a full bridge switching circuit coupled across a primary winding of a transformer and a hybridge rectifier circuit coupled across a secondary winding of the transformer. The transformer is subject to the DC magnetic flux bias as a result of an imbalance in the hybridge rectifier circuit. In one embodiment, the system includes: (1) a sensor configured to develop a signal representing the DC magnetic flux bias in the transformer; and (2) a controller, coupled to the sensor, configured to operate the full bridge switching circuit as a function of the signal thereby to reduce the DC magnetic flux bias.
The present invention, in one aspect, provides a system and method for reducing a DC magnetic flux bias in a transformer of a power converter employing a full bridge switching circuit and a hybridge rectifier circuit. By reducing the DC magnetic flux bias, failure of the power converter due to saturation of a core of the transformer may be avoided.
In one embodiment of the present invention, the sensor includes an integrating differential operational amplifier. The operational amplifier may have resistive inputs coupled across the secondary winding of the transformer. In an alternative embodiment, the sensor includes a first series-coupled resistor and capacitor coupled to an inductor of the hybridge rectifier circuit. The sensor may further include a second series-coupled resistor and capacitor coupled to a second inductor of the hybridge rectifier circuit. The capacitors may be coupled to a stable point. In either case, the signal developed by the sensor may be a function of a magnetic flux in the transformer. By observing the excursions of the magnetic flux in the transformer, the DC magnetic flux bias may be determined and controlled.
In another embodiment of the present invention, the sensor includes a sense resistor coupled in series with the secondary winding. The sense resistor is configured to sense a DC current in the secondary winding. The sensor may thus develop the signal based on the DC current.
In yet another embodiment of the present invention, the sensor includes first and second sense resistors respectively coupled to first and second inductors of the hybridge rectifier circuit. The first and second sense resistors are configured to sense DC currents in the first and second inductors, respectively. The sensor may thus develop the signal based on a difference between the DC currents in the first and second inductors.
In one embodiment of the present invention, the controller is configured to adjust a duty cycle of controllable switches associated with the full bridge switching circuit. By adjusting a duty cycle of one controllable switch relative to another controllable switch, the DC magnetic flux bias may be reduced. In a related embodiment, the controller is configured to reduce a difference between a positive excursion and a negative excursion of a magnetic flux in the transformer. Reducing the difference between the positive and negative excursions reduces the DC bias in the magnetic flux. In another embodiment, the controller employs the signal to terminate a duty cycle of a controllable switch associated with the full bridge switching circuit. Terminating the duty cycle of the controllable switch at an appropriate time may prevent an excursion of the magnetic flux from exceeding a saturation flux of the transformer.
The foregoing has outlined, rather broadly, preferred and alternative features of the present invention so that those skilled in the art may better understand the detailed description of the invention that follows. Additional features of the invention will be described hereinafter that form the subject of the claims of the invention. Those skilled in the art should appreciate that they can readily use the disclosed conception and specific embodiment as a basis for designing or modifying other structures for carrying out the same purposes of the present invention. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the invention in its broadest form.